The present disclosure relates generally to wired and wireless communications systems for a variety of applications but with a particular emphasis on power line communications (PLC).
Link adaptation, or adaptive modulation, or adaptive modulation and coding are terms used in communications to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the wireless or wired link, e.g. path loss, interference due to signals coming from other transmitters, the sensitivity of the receiver, the available transmitter power margin, etc. For example, in a wireless application, enhanced data rated for GSM evolution (EDGE) uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. The process of link adaptation is a dynamic one and the signal and protocol parameters change as the radio link conditions change—for example in High-Speed Downlink Packet Access (HSDPA) in Universal Mobile Telecommunications System (UMTS) this can take place every 2 ms.
Power line networks, particularly, present a hostile channel for communication signals, since their fundamental purpose as the transmission of electric power at super low frequencies. Noise, multipath, selective fading and attenuation are well known properties of power line grids and they should be considered when designing Power Line Communication (PLC) systems. Particularly, random impulsive noise characterized with short durations and very high amplitudes is identified as one of the major impairments that degrade the performance of PLC systems. Orthogonal frequency division multiplexing (OFDM) is the technique of choice for PLC and has been regarded as the modulation scheme for broadband PLC by most researchers. This is because OFDM minimizes the effects of multipath and provides high robustness against selective fading.
Adaptive modulation systems invariably require some channel state information at the transmitter. This could be acquired by assuming that the channel from the transmitter to the receiver is approximately the same as the channel from the receiver to the transmitter. Alternatively, the channel knowledge can also be directly measured at the receiver, and fed back to the transmitter. Adaptive modulation systems improve rate of transmission, and/or bit or packet error rates, by exploiting the channel state information that is present at the transmitter. Adaptive modulation systems exhibit great performance enhancements compared to systems that do not exploit channel knowledge at the transmitter.
A conventional communication system will be described using FIG. 1 through FIG. 4.
FIG. 1 shows system 100, which illustrates the components of a basic two way communication system.
As shown in the figure, system 100 includes a transmitter 102 a receiver 104 and a transmission line 106.
Transmission line 106 is arranged between transmitter 102 and receiver 104. Transmitter 102 is arranged to receive signals on an input 108. Receiver 104 is arranged to send received signals on an output 110.
Transmitter 102 is operable to modulate, upconvert and transmit signals across transmission lines. Receiver 104 is operable to receive, downconvert and demodulate signals arriving on transmission lines. Transmission line 106 is operable to be the medium for propagation of communications signals between transmitters and receivers either wirelessly or using a wire.
In operation, transmitter 102 receives a signal from input 108 and modulates the signal on to a carrier frequency. Transmitter 102 may also upconvert the modulated carrier frequency to a higher frequency for transmission over transmission line 106. Receiver 104 downconverts the modulated carrier received from transmission line 106 to a lower frequency and demodulates the modulated signal for output on output 110.
A particular application of the basic transmission line system of system 100 is Power Line Communications (PLC) where power lines, used primarily to deliver electrical power, are also utilized as transmission lines for communications signals.
FIG. 2 illustrates system 200, a PLC application of system 100.
As shown in the figure, system 200 includes a generator 202, a line 204, a transmission line 206, a transmitter 208, a receiver 210, a line 212 and a line 214.
Line 204 is arranged to connect generator 202 to transmission line 206. Transmission line 206 is arranged between transmitter 208 and receiver 210 via line 212 and line 214.
Generator 202 is operable to deliver electrical power across transmission lines. Transmitter 208 is operable to modulate, upconvert and transmit signals across transmission lines. Receiver 210 is operable to receive, downconvert and demodulate signals arriving on transmission lines. Transmission line 206 is operable to be the wired medium for propagation of both electrical power and communications signals.
In operation, generator 202 is generating electrical power to transmission line 206 for distribution. At the same time, transmitter 208 modulates communications signals on to a carrier frequency. Transmitter 208 upconverts the modulated carrier frequency to a higher frequency for transmission over transmission line 206 via line 212. Receiver 210 downconverts the modulated carrier received from transmission line 206 via line 214 to a lower frequency and then demodulates the modulated signal. Modulation and demodulation, in this embodiment, may also include forward error correction (FEC) encoding and decoding.
A conventional PLC system, as illustrated by system 200, may be used in applications such as smart utility metering, automated meter reading, renewable energy communications, lighting control, electric vehicle servicing, etc. and as such may comply with the most well-known standards, namely PRIME, ITU G9903 and IEEE 1901.2. These standards address the harsh channel conditions encountered by data communications across noisy power lines. PRIME, ITU G9903 and IEEE 1901.2 use OFDM for physical layer communication. Modulation schemes used within the OFDM framework can be BPSK, ROBO/BPSK (or super ROBO), QPSK, 8-PSK, and in some cases 16-QAM. Each of these modulation schemes has a different signal-to-noise (SNR) requirement in order to satisfy a target frame-error-rate figure. The slowest data rate modulation (ROBO/BPSK) has the least stringent SNR requirement while the highest data rate (8-PSK for most systems) has the most stringent, the difference between these being as much as 15 dB.
For a conventional system, always supporting the highest data rate possible is the goal. To achieve this in a point-to-point link, the transmitter periodically sends “sounding” packets to the receiver. Using the sounding packets, the receiver estimates the SNR of the channel and communicates back to the transmitter the fastest modulation it can support given that estimate. The transmitter will then synchronously change modulations so that the highest data rate is always maintained. This method is known as ‘adaptive modulation”. The receiver also has the capability to send requests to the transmitter for it to adjust the transmit RF power. Increasing transmit RF power is another way of increasing the SNR, a method which can be used alongside modulation changes to maximize data rate.
FIG. 3 shows system 300, a conventional system which adapts its modulation scheme to transmission line conditions in order to maintain the highest data rate.
As shown in the figure, system 300 includes transmission line 106, a modulating component 302, a transmitting component 304, a receiving component 306, a demodulating component 308 and a modulation controller 310.
Modulating component 302 is arranged between input 108 and transmitting component 304 via a line 312. Transmitting component 304 is arranged to connect to transmission line 106 via a line 314, while receiving component 306 is arranged to connect to transmission line 106 via a line 316. Demodulating component 408 is arranged to connect between receiving component 306 via a line 320 and modulation controller 310 via a line 322. Demodulating component 308 also connects to an output 324. Modulation controller 310 connects to modulating component 302 via a line 318.
Modulating component 302 is operable to modulate input signals onto a carrier frequency. Modulating component 302 is also operable to support several modulation schemes. Modulating component 302 receives data from input 108 and creates modulated packets in accordance a modulation for which it is currently operating.
Transmitting component 304 is operable to upconvert a carrier frequency to a higher carrier frequency and to transmit a carrier over a transmission line. Transmitting component 302 modulated packets from modulating component 302 and creates transmit packets for transmission through channel 106.
Receiving component 306 is operable to receive a carrier frequency from a transmission line and to downconvert the carrier frequency to a lower carrier frequency. Demodulating component 308 is operable to demodulate a carrier frequency to provide an output signal. Demodulating component 308 is also operable to support several modulation schemes. Modulation controller 310 is operable to receive information from a demodulator and to send a control signal to a modulator.
System 300 represents the near end of a two-way communications link across a transmission line. The far end of the link duplicates the near end is also represented by system 300, but is not shown.
In operation, the data signal appearing on input 108 is modulated on to a carrier by modulating component 302 to create a stream of modulated packets. Transmitting component 304 creates a stream of transmit packets from the stream of modulated packets. Transmitting component then transmits the stream of transmit packets over transmission line 106 by transmitting component 304 to the far end receiver. Receiving component 306 receives a signal from the far end's transmitter and the signal is demodulated by demodulating component 308 and output via line 324. The output signal on line 324 is tapped off and sent to modulation controller 310 via line 322. Modulation controller sends commands to modulating component 302 via line 318.
System 300 adjusts its modulation scheme to maintain the highest data rate possible for channel 106 to achieve during operation.
In some systems, modulating component 302 knows parameters of channel 106, e.g., via a priori SNR information of channel 106. Modulation controller 310 uses the known channel SNR information to calculate the highest viable modulation scheme which can be used by the system. Modulation controller 310 then sends commands to modulating component 302 to update its modulation scheme. Thus, the highest data rate possible for the channel is always maintained during operation. In this example, transmitted RF power is maintained at a constant level, conventionally the highest level possible for the conditions (and any regulations) since this ensures the highest SNRs possible.
In some other systems, modulating component 302 sends sounding packets to the far end receiver which uses these packets to estimate the SNR of the channel sent over transmission line 106 and sends the SNR estimation back. This is received by receiving component 306 and modulation controller 310, which, since it can see the received signal, extracts the SNR information and uses it to calculate the highest viable modulation scheme which can be used by the system. Modulation controller 310 then sends commands to modulating component 302 to update its modulation scheme. Thus, the highest data rate possible for the channel is always maintained during operation. In this example, transmitted RF power is maintained at a constant level, conventionally the highest level possible for the conditions (and any regulations) since this ensures the highest SNRs possible.
Data is sent over the transmission channel using packets. Data packets can also be referred to as frames.
FIG. 4 shows an example transmit packet 400, which illustrates the basic structure of a packet sent over system 300.
As shown in the figure, transmit packet 400 includes a preamble section 402, a header section 404 and a data section 406. Transmit packet 400 corresponds to a transmit packet created by transmitting component 304.
The sections are arranged in time in the order shown in the figure with preamble section 302 being transmitted first.
Preamble section 402 is a series of bits or symbols intended to allow the far end receiver to detect the arrival of the beginning of a packet and to synchronize to it. Header section 404 is a series of bits or symbols to convey information about the packet to the far end receiver such as frame length, modulation, control check data, etc. Data section 406 is the data field intended to be transmitted. This data section corresponds to the modulated packet created by modulating component 302. As shown encompassed by a bracket 408, preamble section 402 and header section 404 are the overhead portions of the packet, and as shown by bracket 410, data section 406 is the payload portion. Conventionally, the overhead portion is of fixed length while data length can be of variable length.
Transmitting component 304 creates preamble section 402 and header section 404. Transmitting component 304 additionally adds preamble section 402 and header section 404 to data section 406 to create transmit packet.
It was stated earlier, that in order to maximize data throughput, a conventional system will tend to use the highest rate modulation scheme which is viable transmitted at the highest viable RF power. However, this basis for modulation and power selection can create problems in applications where power consumption is very important, such as in battery powered equipment applications or in other applications where available energy is limited, solar powered equipment for instance. In such systems, data throughput may have to take a back seat to energy conservation. Conventional systems, however, do not operate this way.
What is needed, therefore, is a system and method which can control modulation choice and RF transmit power based on minimizing the energy consumed by the link rather than on maximizing the data speeds.